1. Field of the Invention
The present invention relates to an improved device to detect magnetic resonance in the time domain. This would lead to an optimal receiver for detecting magnetic resonance signals.
2. Description of Related Art
Time Domain Magnetic Resonance signals are excited by bursts or pulses of resonant radiofrequency (RF) or microwave radiation and subsequently detected by phase sensitive receivers. These are then Fourier transformed to produce the corresponding frequency spectra. In operating in pulsed mode, two basic factors need to be kept in view:
(i) Efficient use of available transmitter power; and PA1 (ii) Efficient use of detector bandwidth.
The first factor implies that one normally prefers to operate the transmitter from the middle of the spectral window of interest, so that the transmitter field, symmetrically irradiates spectral features on either side of its frequency. The second factor implies that one normally prefers to operate the detector (receiver) from the middle of the spectral window as well, so that the detector filter bandpass just covers the desired spectral window.
Following demodulation by phase sensitive detection (psd), signals that occur originally at distinguishable frequencies (.omega.+.DELTA.) and (.omega.-.DELTA.) are down-converted to frequencies (.+-..DELTA.) which are identical in magnitude and differ only in sign. In meeting the objective of efficient excitation, therefore, the need arises to distinguish signals at equal but opposite frequencies. This problem does not arise when the transmitter is positioned to one side of the spectral window, so that all signal frequencies are to one side of the transmitter frequency .omega.. However, this mode of operation, which is standard in single detection mode, is wasteful of the available transmitter power. The problem of frequency sign discrimination has been solved by quadrature detection. When quadrature detection is employed, a given transmitter power level may be used to irradiate twice the spectral width that would be possible in single detection mode. At the same time, the receiver filter bandwidth may be reduced by a factor of 2 compared to single detection, resulting in noise reduction by a factor of .sqroot.2.
The essential idea in quadrature detection is to detect two components of the signal that are in phase quadrature with each other (eg. the sine and cosine components), so that the desired frequency sign discrimination is achieved, eg. by a complex Fourier transformation. The standard realization of such a system involves the implementation of two identical receiver channels that get reference inputs which are in phase quadrature with each other and simultaneously detect two signals that are in phase quadrature and are paired to form a complex signal. Such a system typically involves two receiver and digitizer channels. A practical limitation of this approach is that a mismatch in the phase and amplitude characteristics of the two channels results in the generation of artifacts known as image peaks. Image peaks are minimized by a careful adjustment and matching of the two receiver channels, and are further suppressed by special phase cycling procedures such as the Cyclically Ordered Phase Sequence (CYCLOPS). A block diagram of this general approach is shown in FIG. 1 of the drawings accompanying this specification.
It essentially consists of two Phase Sensitive Detectors (5, 8), two filters (6, 9) and two digitizers (7, 10), one set for each receiver channel. An RF source (1) feeds a transmitter (2) and a phase splitter (4); the transmitter (2) delivers pulses to a probe-preamplifier (3). The probe houses the sample whose Magnetic Resonance is being investigated. The signal output of (3) is fed to (5) and (8), which get reference inputs from (4) which are in phase quadrature. The output of (5) and (8) is filtered by (6) and (9) and then digitized by (7) and (10) before finally feeding to the computer (11).
Single detection with crystal filter has also been developed as an alternative to quadrature detection. The block diagram of this scheme is shown in FIG. 2 of the drawings accompanying this specification and essentially comprises an RF source (12) that feeds a transmitter (13) and a Phase sensitive Detector (16); the transmitter (13) delivers pulses to a probe-preamplifier (14). The probe houses the sample whose Magnetic Resonance is being investigated. The signal output of 14 is fed to the crystal filter (15), then to 16 and finally to the Digitizer (17) and Computer (18). This scheme typically gains the .sqroot.2 sensitivity advantage of quadrature detection by avoiding noise foldover from the side that is opposite to the spectral window with respect to the receiver reference frequency; however, this scheme cannot avoid positioning the transmitter at one end of the spectral window. The typical crystal filter is a four-pole Butterworth crystal filter, with Quality-factor Q of the order of 10.
Yet another scheme acquires quadrature phase shifted signals on alternate scans in a single detection receiver--by phase shifting the excitation pulse appropriately--and synthesizes the quadrature information by pairing together successive scans to produce a complex signal. Sensitivity is lost in this implementation, while the transmitter may be positioned in the center of the spectral window. A schematic diagram of this implementation is shown in FIG. 3 of the drawings accompanying this specification and essentially comprises an RF source (19), phase splitter (20), transmitter (21), probe-preamplifier (22), Phase Sensitive Detector (23), Filter (24), Digitizer (25) and computer (26).
Still another scheme uses two quadrature phase receivers, but generates a single real signal by routing alternate points of the signal respectively into the two receiver channels, accompanied by sign alternation in the computer after every two sampling points. This scheme, which is shown in FIG. 4 of the drawings accompanying this specification, also suffers from the mismatch of the two receiver channels noted above, resulting in image artifacts. Various components of the device, as shown in FIG. 4 with numerals, are described below:
27 refers to an RF source. PA0 28 refers to the transmitter. PA0 29 refers to the probe-preamplifier. PA0 30 refers to the phase splitter. PA0 31 and 34 refer to Phase Sensitive Detectors. PA0 32 and 35 refer to Filters. PA0 33 and 36 refer to Digitizers. PA0 37 refers to the Computer.
Yet another scheme uses a single receiver channel whose phase reference is incremented by 90.degree. for every successive point of the signal that is acquired. This last scheme, shown in FIG. 5 of the drawings accompanying this specification, essentially comprises an RF source (38), transmitter (39), probe-preamplifier (40), a phase splitter (41) with four reference frequency outputs of phase 0.degree., 90.degree., 180.degree., and 270.degree. respectively, a Phase sensitive Detector (42), Filter (43), Digitizer (44) and computer (45). This modus operandi also gives rise to artifact image peaks arising from the continual jump in receiver reference phase. Further, it suffers from a loss of signal detection sensitivity, owing to the periodic discrete phase jumps that result in sidebands.
Almost all practical quadrature detection systems that are currently in use employ one of the two two-channel receiver/digitizer implementations. Most recently, two-channel receiver/digitizer systems are employed with oversampling followed by signal decimation and digital filtering to produce quadrature detected spectra with improved dynamic range and baseline characteristics. The two-channel receiver systems require careful matching of the two receiver channels, as any mismatch results in artifacts. The single channel receiver systems is susceptible to either low sensitivity or low excitation efficiency or artifacts due to phase splitters.